The present invention relates to AC-coupled fiber optic receivers and more particularly to an AC-coupled fiber optic receivers that do not require message preambles for proper operation.
Optical receivers translate pulses of optical energy into equivalent electrical signals. The optical pulses produce small currents in a photosensitive semiconductor, which are amplified and processed by electronic circuits to produce an electrical signal that duplicates the optical pulses.
The signals can be of two general types: DC-coupled and AC-coupled. DC-coupled circuits respond to the magnitude of the optical signals no matter how much time elapses between signal transitions. These circuits are always prepared to translate the magnitude of the optical input to a high or low logic value. This feature is desirable when processing data that comes in unpredictable bursts. The major disadvantage of DC-coupled circuits is that their signals drift with temperature and age. They are especially difficult to use when high amplification is necessary.
Most optical receivers use AC-coupled circuits which cannot respond to signals that change too slowly. AC-coupled circuits do not suffer the disadvantages of signal drift with temperature and age that the DC-coupled circuits are subject to, but the AC-coupled circuits are not always prepared to process data. AC circuits need time to settle and stabilize after the input signal begins before they can properly interpret the data. To compensate for this characteristic of AC-coupled receivers, data is prefixed with a "preamble" string of pulses that prepares the receiver to receive data.
It is an object of the present invention to provide an AC-coupled fiber optic receiver that does not require a preamble before the data to be processed.
It is another object of the present invention to provide an AC-coupled fiber optic receiver that is easy to use with any two-level data format independent of the time between optical signal transitions.
It is a further object of the present invention to provide an AC-coupled fiber optic receiver with an increased signal to noise ratio.